In numerous mechanical and aeronautical applications it is desirable to control the flow of fluid across a surface. In conventional airplanes, for example, the air flowing above and below the wing at different speeds creates the lift necessary to raise or elevate the plane off the ground. The curvature of the upper surface of the wing causes the air to flow across the surface of the top of the wing at a speed faster than the speed of the air flowing across the bottom of the wing. The faster air flow across the top surface of the wing creates a reduced pressure region along the top surface of the wing. A high pressure zone is created along the bottom side of the wing, due to the generally flat lower surface, thereby generating a net upward force.
As the angle of attack of an airplane wing is increased, such as for example during a steep takeoff, there is a tendency of the air flow passing across the top surface of the wing to become destabilized and separate from the wing. This flow separation could bring disastrous results because the reduced pressure zone on the wing top surface is diminished, and the lift is dramatically reduced while the drag is substantially increased. Accordingly, it is beneficial to ensure that the air flow not separate from the top surface of the wing, or at least if flow separation occurs there is a mechanism to reattach the flow to the wing surface. Another area where it is necessary for an air flow to be maintained in contact with a surface is in jet engines. A generally attached flow along the surface of the air intake is preferred for a stable operation.
There have been past attempts to ensure that the flow of air does not separate from the surfaces of a airfoil. For purposes of this invention, an airfoil is defined as a part or surface of an aircraft, such as a wing, aileron or rudder, vertical tail, horizontal tail, and the like, whose shape and orientation control the stability, direction, lift, thrust or propulsion of an aircraft by virtue of the flow of fluid, such as air, with respect to the airfoil. It is known that increasing the turbulence on the top surface of the wing can reduce the tendency of the air flow to separate from the wing top surface. Mechanical vortex generators are currently used on airplanes to increase turbulence on the top surface of the wing and thereby enable a higher angle of attack without separation. See, for example, U.S. Pat. No. 5,755,408 to Schmidt et al., which the raising of a boundary layer penetrator generates vortices capable of reattachment of the air flow. Unfortunately, mechanical vortex generators impose a significant air drag to the plane during cruising. Retractable mechanical vortex generators have been considered, but they are relatively slow in response, and can impose a significant weight penalty.
Past attempts to more efficiently create upper wing surface vortices for increased flow control include pneumatic vortex generators based on both suction and blowing. It has been proposed to use an array of uniform suction ports along the leading edge of a wing to try to control air flow separation from the wing, but this apparatus does not generate vortices. An improvement in suction control was disclosed in a paper entitled "Separation Control Through the Translative Instability", by the present inventor. This paper sets forth an air flow control apparatus using an array of vortex generators spaced along the leading edge of the wing. Each of the vortex generators consisted of a generally triangular shaped region of suction arrays consisting of small holes all connected to a suction manifold. The vortices generated by each of the suction arrays help the flow of air stay connected to the top surface of the wing.
One of the shortcomings of the apparatus using triangular shaped suction arrays of small holes described above is the fact that the suction arrays are fixed in place, and cannot easily be controlled to accommodate changing conditions during operation of the aircraft. It would be beneficial if there could be developed an airfoil vortex generating system that provides better control of each vortex generated by the apparatus. Further, it would be advantageous if the apparatus could be self cleaning, and could be adapted with sensors and controllers to enable feedback for controlling the operation of the vortex generators and the reattachment of the air flow along the top of the wing. Optimally, such system will present a hydrodynamically smooth surface for minimal drag and for reducing the unwanted generation of turbulence in the air flow when such turbulence is not required. Preferably, the system could be adapted for use on aircraft having moderate leading edge sweeps as well as on conventional aircraft having low leading edge sweeps.